The present invention relates to a system for transmitting electric power comprising a direct voltage network, and at least two alternating voltage networks which are connected to the direct voltage network through a respective power station. The power stations transfers power between the direct voltage network and the respective alternating voltage network, and has at least one VSC converter to convert direct voltage into alternating voltage, and conversely convert alternating voltage to a direct voltage. An apparatus in a first station controls the converter of the station to regulate the direct voltage of the direct voltage network and maintains the direct voltage at a predetermined nominal value. An apparatus of the other station controls the converter of that station for regulating the current flowing between the respective connected alternating voltage network and its connected station.
Such a system is known through the thesis "PWM and Control of Two and Three Level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, which describes a system for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC). It is emphasized that the invention is not restricted to this application, but for purposes of illustration the invention will be described with respect to this known system.
Prior to the system described in the thesis, systems for transmitting electric power through a direct voltage network for High Voltage Direct Current have been based upon the use of line-commutated CSC (Current Source Converter) converters in the power stations. The development of IGBTs (Insulated Gate Bipolar Transistor=bipolar transistor having an insulated gate) for high voltage applications which may be easily turned on and turned off simultaneously, has made it possible to create valves for VSC (Voltage Source Converter) converters. VSC converters permit forced commutation as an alternative. This type of power transmission between a direct voltage network for High Voltage Direct Current and alternating voltage networks connected thereto offers several important advantages over the use of the prior art line-commutated CSCs used in HVDC. Th VSC (Voltage Source Converter) permits the consumption of active and reactive power to be controlled independently of each other, and there is no risk of commutation failures in the converter and no corresponding risk of transmitting commutation failures between different HVDC links, which may take place in line-commutated CSC. Furthermore, there is the possibility of feeding power to a weak alternating voltage network or a network which doesn't generate its own power (a "dead" alternating voltage network).
In a system of the type discussed above for HVDC with VSC converters, the direct voltage of the direct voltage network is determined in a first power station, and it is desirable to control the direct voltage level without any rapid telecommunication between a station having voltage-regulating apparatus and the other stations along the direct voltage network. Since each power unbalance results in a change of the direct voltage, which is then corrected by the voltage-regulating station, such a communication possibility will be superfluous. Should the power fed into the network drop as a consequence of limitations in an alternating voltage network for feeding power to a direct voltage network, or in the voltage-regulating station, or should the power fed out of the network exceed the available power fed in, the direct voltage of the direct voltage network will drop. The direct voltage will drop so much that the power fed out is reduced to such a level that power balance is reestablished. This means that the voltage-regulating station has arrived at its regulation limit and cannot maintain the direct voltage at said determined nominal value. This may result in collapse of the direct voltage and temporary disabling of parts of the HVDC system.